1. Field of the Invention
The present invention relates to a centrifugal fan, and more particularly to a centrifugal fan, an expansion angle of which varies without increasing the overall width of a scroll housing, thereby improving blowing capacity and reducing noise.
2. Description of the Related Art
Generally, a centrifugal fan for emitting heat, which is referred to as a “sirocco fan”, is widely used by household electric appliances including an LCD projector. As shown in FIG. 1, the centrifugal fan comprises an impeller 11 rotated by a motor, and a scroll housing 12 for guiding air inhaled by the impeller 11 to an outlet 12b to discharge the air to the outside.
The impeller 11 includes a rib 11b, and a plurality of blades 11a supported by the rib 11b, and is connected to an actuating unit of the motor. The scroll housing 12 is designed such that air is inhaled thereinto through an inlet 12a formed through the front surface thereof by the guide of a bell mouth 13, and is then discharged to the outside through the outlet 12b along a path expanded from a cutoff portion. That is, when the impeller 11 connected to the actuating unit is rotated, air is inhaled into the scroll housing 12 through the inlet 12a, travels along the gradually expanded path of the scroll housing 12, and is discharged to the outside through the outlet 12b. 
Here, since noise and flow rate generated from the centrifugal fan 10 are varied according to the design of the scroll housing 12, a design of the scroll housing having low noise and high flow rate has been developed.
In FIG. 1, θ0 represents a reference angle of a portion where a curved surface forming the outer periphery of the scroll housing 50 is finished, θc represents a position angle of the cutoff portion (C), and θx represents an angle of rotation of the impeller 11 from the reference angle (θ0) in a counterclockwise direction.
FIG. 2 is a graph illustrating an expansion angle of a conventional centrifugal fan, a scroll housing of which is designed using an Archimedean scroll curve. FIG. 3 is a schematic front view of the conventional centrifugal fan, the scroll housing of which is designed using the Archimedean scroll curve. FIG. 4 is a graph illustrating an expansion angle of another conventional centrifugal fan, a scroll housing of which is designed using an exponential scroll curve.
As shown in FIGS. 2 and 4, the scrolling housings 12 of the conventional centrifugal fans are divided into two types, i.e., one type which is designed using the Archimedean scroll curve (A) and the other type which is designed using the exponential scroll curve (B).
First, with reference to FIGS. 2 and 3, a method for designing the outer diameter of the scroll housing 12 using the Archimedean scroll curve (A) will be described. The scroll housing 12 has a structure such that the radius (Rθ) of curvature of the scroll housing 12 is proportionate to angles (θ) based on a mean velocity formula when the radius (R0) of the impeller 11 is determined. In case that the expansion angle of the scroll housing 12 is represented by α, the radius (Rθ) of curvature of the scroll housing 12 at a designated angle (θx) is calculated by The Equations below.
            tan      ⁡              (        α        )              =          (                                    R            θ                    -                      (                                          R                0                            +                              C                C                                      )                                    2          ⁢                      π            ⁡                          (                                                R                  0                                +                                  C                  C                                            )                                ⁢                      (                                                            θ                  x                                -                                  θ                  c                                            360                        )                              )                  R      θ        =                  (                              R            0                    +                      C            C                          )            +                        tan          ⁡                      (            α            )                          ⁢                  (                      2            ⁢                          π              ⁡                              (                                                      R                    0                                    +                                      C                    C                                                  )                                      ⁢                          (                                                                    θ                    x                                    -                                      θ                    c                                                  360                            )                                )                                R      θ        =                  (                              R            0                    +                      C            C                          )            +              (                  1          +                                    tan              ⁡                              (                α                )                                      ⁢                          π              ⁡                              (                                                                            θ                      x                                        -                                          θ                      c                                                        180                                )                                                    )            
Here, R0 represents the radius (mm) of the impeller 11, θx represents a designated angle (°), CC represents the cleavage (mm) of the cutoff portion, and θc represents the position angle (°) of the cutoff portion.
Thereafter, with reference to FIG. 4, a method for designing the outer diameter of the scroll housing 12 using the exponential scroll curve (E) will be described. The scroll housing 12 has a structure such that the radius (Rθ) of curvature of the scroll housing 12 is exponentially increased based on a free vortex formula. In case that the expansion angle of the scroll housing 12 is represented by α, the radius (Rθ) of curvature of the scroll housing 12 at a designated angle (θx) is calculated by the Equation below.
      R    θ    =            (                        R          0                +                  C          C                    )        ×          ⅇ              (                              tan            ⁡                          (              α              )                                ⁢          π          ⁢                                                                                ⁢                                                θ                  x                                -                                  θ                  c                                                      180                          )            
Here, in the Archimedean scroll curve (A) as shown in FIG. 2, the width (W) of the scroll housing 12 is the sum total of the width (w180) of the scroll housing 12 when the radius (Rθ) of curvature thereof is 180° and the width (w360) of the scroll housing 12 when the radius (Rθ) of curvature thereof is 360°. Accordingly, when the radius (R0) of the impeller 11 is determined and the width (W) of the scroll housing 12 is constant, the expansion angle (α) of the scroll housing 12 is restricted by the above-described Equations.
That is, in case that the radius (R0) of the impeller 11 is set to 40 mm, the cleavage (CC) of the cutoff portion is set to 5 mm, the position angle (θc) of the cutoff portion is set to 90°, and the width (W) of the scroll housing 12 is set to 115 mm, the maximum expansion angle (α) of the scroll housing 12 designed using the Archimedean scroll curve (A) is 5.053°, w180 is 51.2501 mm, and w360 is 63.7503 mm.
On the other hand, the maximum expansion angle (α) of the scroll housing 12 designed using the exponential scroll curve (E) is 4.3334°, w180 is 50.6882 mm, and w360 is 64.3123 mm.
Since the maximum expansion angle (α) of the scroll housing 12 of the conventional centrifugal fan is constant when the radius (R0) of the impeller 11 and the cleavage (CC) of the cutoff portion are determined and the width (W) of the scroll housing 12 is constant, the radius (R0) of the impeller 11 and the cleavage (CC) of the cutoff portion of the scroll housing 12 of the conventional centrifugal fan must be reduced in order to increase the expansion angle (α), which affects the flow rate. However, this design causes problems, such as the reduction of blast capacity and the increase of noise.